It is important to grasp a motor constant such as an armature resistance or an inductance so as to control a permanent magnet type synchronous motor with high accuracy. Under, for example, a position sensorless control in which the above-described motor is controlled without a magnetic pole position sensor, a technique for estimating a magnetic pole position with a motor constant is adopted in many cases. Alternatively, a current is controlled in most motors under a vector control, wherein a motor constant is used to properly set a gain in a current controller.
In order to readily obtain a motor constant in the above-described motor, there has been known some techniques for obtaining the motor constant without rotating the motor. Patent Literature 1, for example, has been known. In the technique disclosed in Patent Literature 1, a DC current is allowed to flow, and then, a coil wire resistance is obtained based on an input voltage and an input current at that time. At the same time, fundamental wave components of an input voltage and an input current when an AC current is allowed to flow are extracted, and thereafter, an inductance is obtained based on the value of each of the voltage and the current and a difference in phase therebetween.
Moreover, Patent Literature 2 has been known as another technique. In this technique, first, a voltage is stored when DC currents on a higher level and a lower level are allowed to flow. A difference in voltage between the two levels is divided by a difference in current between the two levels, thereby calculating a coil wire resistance. At the same time, the voltage is sharply changed from a voltage value based on the higher current level to a voltage value based on the lower current level. A period of time until the current at the time of the sharp change is changed to a predetermined value is measured, so that an inductance is calculated based on the resultant period of time and the coil wire resistance.
However, the technique disclosed in Patent Literature 1 requires a time until the DC current becomes constant. Additionally, since different test signals are used for measuring the resistance and the inductance, a time is required for the measurement. In addition, unless the frequency of the AC current which is allowed to flow for obtaining the inductance is properly set, the detection accuracy of the difference in phase is degraded, resulting in a large error in a measurement result.
The technique disclosed in Patent Literature 2 also requires a time until the DC current is allowed to flow to become constant. Additionally, since different test signals are used for measuring the resistance and the inductance, a time is required for the measurement. Furthermore, in the case of measurement of a period of time until the current is changed to a predetermined value after the sharp change of the voltage value, level judgment or time measurement is liable to be erroneous. Moreover, in the case of a motor whose inductance value is changed according to the current value, a response having a predetermined time constant cannot be obtained, and therefore, an inductance value cannot be correctly calculated.